Just messing around with the settings, I thought I’d look at the other (default) theme available in Preferences: Interface, with every intention of changing it back. Only changing it back wasn’t so simple! A huge Patreon logo appeared over the controls, making them unusable! This on the latest Firefox.
It only seems to be recoverable by using web developer tools to select the div with the logo in it and delete it from the loaded page.
(Hoping this goes into the right place!)